In an important aspect, this invention is an improvement on that described in my U.S. Pat. No. 5,549,251. The new rotating throat described and claimed herein has significantly less mass than previous throats, replaces the lower part of the prior art throat with a series of vanes of a particular shape, and provides an overall restricted air passage area through the throat in order to generate a high pressure drop rather than a low pressure drop as is conventional.
The present invention applies to various types of roll-and-ring grinding mills which are air swept and utilize a vertical spindle. Roll-and-ring grinding mills have a circular track, groove or grinding table in which the heavy rolls contact the coal. Surrounding the grinding table is a series of air ports collectively known as a throat, through which air is forced to move the particulate coal upwards to a classifier.
As mentioned in my earlier U.S. Pat. No. 5,549,251 (col. 2 lines 24-34), primary air performs four functions in the pulverizer : (1) drying of the coal in the pulverizer, (2) maintaining a fluidized bed of coal, which circulates coal into the path of the grinding elements, (3) transporting the coal particles from the fluidized bed into the classifier assembly, where large particles are separated for return to the grinding elements, and (4) transporting suitably pulverized coal particles out of the classifier to the burners.
Separation of the smaller and larger coal particles and recycling of the larger ones is common to most if not all pulverizer designs. The task is complicated, however, by the presence of relatively dense non-combustible materials, i.e. rock which is incidentally introduced as part of the coal feed. Where the machine functions actually to recycle such materials rather than separating them out, the inefficiency is manifest. The machine not only expends unnecessary energy on recirculating and regrinding a material of zero fuel value, but does so at the cost of considerable wear. If somehow the rock particles are not rejected from the coal being processed, they must be reduced in size until they can mix with the coal particles transported to the burners. The presence of rock particles in the fuel stream reduces combustion efficiency and also results in a greater and faster buildup of ash in the combustion chamber, further reducing boiler efficiency by retarding heat transfer from the combustion chamber.
Kitto and Kowalski, in U.S. Pat. N0. 4,264,041, describe a construction of a Babcock and Wilcox pulverizer throat said to provide a low pressure drop, which they associate with reduced erosion and improved air flow distribution. They use a particular ratio of curvatures of the inlets and outlets of the throat to the radial width of the throat to effect a reduction or deceleration of air flow through the outlet of the throat in order to "minimize dribble" (claim 1) of solid material through the throat. The Kitto et al construction and other similar designs providing low pressure drops particularly in the throat outlet are widely used but have demonstrated difficulty in controlling foreign substances such as rock which can cause excessive erosion of the pulverizer parts and reduced efficiency due to the unwanted processing of the rock particles. In general, throats with low pressure drops fail to create a jet of air sufficiently energetic to prevent coal from being rejected and thus require volumes of air greater, frequently substantially greater, than the typical nominal 2:1 weight ratio to coal.